Fuel injection control device for internal combustion engine

Abstract: An excess voltage applied to a piezoelectric element is detected and controlled to improve durability and reliability of the piezoelectric element. A fuel injection control device for an internal combustion engine in which a power source voltage is boosted by a dc-dc converter (1) and the so-boosted output voltage is supplied via a first choke coil (Lc) and a first thyristor (SCR1) to a piezoelectric element (PZT) for charging the piezoelectric element (PZT) for being subsequently discharged via a second thyristor (SCR2) and a second choke coil (Ld). The fuel injection control device includes a positive voltage detection unit (10) for detecting the positive voltage at a junction between the first choke coil (Lc) and the first thyristor (SCR1) and a positive voltage control unit (20) controlled by an output signal of the positive voltage detection unit (110) and adapted for shorting both ends of the first choke coil (Lc). The current through the first thyristor (SCR1) is cut off to prevent the excess voltage from being applied to the piezoelectric element (PZT).

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a fuel injection control device for an internal combustion engine. More particularly, it relates to a fuel injection control device in which a piezoelectric element is employed in a fuel injection valve for taking advantage of shape changes in the piezoelectric element caused by voltage application thereto. Related Art It has hitherto been known to use a piezoelectric element for a fuel injection valve for controlling the fuel injection. For example, the JP Patent KOKAI Publication No.64-69756 (1989) discloses means for stabilizing changes in capacitance of a piezoelectric element caused by changes in temperature. The piezoelectric element is increased in capacitance with the rise in temperature. If the capacitance is increased, the piezoelectric element is deformed to a larger extent for the same applied voltage because the deformation in the piezoelectric element is proportionate to the amount of the stored electric charge which is proportionate to the capacitance value. The above cited publication proposes a fuel injection control device equipped with means for detecting the change in temperature of the piezoelectric element and accordingly controlling the voltage applied to the piezoelectric element for controlling a displacement of the piezoelectric element. DISCUSSION OF THE RELATED ART However, there lacks up to now a proposal for controlling an excess voltage or a negative voltage applied to a piezoelectric element employed as a fuel injection valve. The piezoelectric element suffers from the drawback that it is deteriorated or destroyed under a voltage higher than its withstand voltage, and that it is susceptible to deterioration due to a polarization when under an excessive negative voltage and thereby is lowered in reliability or durability. Document EP 0 464 443 discloses a device for driving a piezoelectric element, wherein a power condenser for charging the piezoelectric element and a first switching means connected between the power condenser and the piezoelectric element is provided. The piezoelectric element is charged by using a high potential at a high voltage side terminal of the power condenser. A second switching means is arranged between the piezoelectric element and the ground terminal for discharging the piezoelectric element to the ground via a first discharging coil, whereas a third switching means is connected between the piezoelectric element and the high voltage side of the power condenser for discharging the piezoelectric element to the high voltage side of the power condenser via a second discharging coil. When the discharging operation of the piezoelectric element is started, it is under control of a control means initially discharged to the high voltage side terminal of the power condenser via the second discharging coil and is thereafter discharged to the ground via the first discharging coil. After a completion of the discharging operation and before a new charging operation, the negative voltage at a terminal of the piezoelectric element (which has dropped to a minimum) is detected by a detecting means and the start time of the discharge to the ground is controlled so that the minimum voltage becomes equal to a predetermined negative voltage. SUMMARY OF THE DISCLOSURE For accomplishing the above object, the present invention provides a fuel injection control device for an internal combustion engine as claimed in claim 1. SUMMARY OF THE PREFERRED EMBODIMENTS With a fuel injection control device for an internal combustion engine of the present invention, if a desired voltage close to a withstand voltage is applied to a piezoelectric element, an output signal from a positive excess voltage detection unit actuates a positive excess voltage control unit to short both ends of a first choke coil. This causes the current flowing from the first choke coil to the first thyristor to flow in the positive voltage control unit. As a result thereof, the current through the first thyristor is cut off to turn the first thyristor to an OFF state to prevent a voltage in excess of the withstand voltage from being applied to the piezoelectric element. According to the present invention, as described, since the positive voltage applied to the piezoelectric element may be controlled within the constant range, there is no risk of application of a voltage in excess of a withstand voltage. Since the excess voltage is detected at the thyristor terminals, the piezoelectric element is not effected by leakage or the like. Consequently, there is no risk of the piezoelectric element being destroyed or lowered in properties, while the piezoelectric element may be improved in durability and operational reliability. BRIEF DESCRIPTION OF THE DRAWING Fig.1 illustrates an embodiment of the present invention having an excess positive voltage detection control circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, preferred embodiments of the present invention will be explained in detail. Embodiment 1 Fig.1 shows an embodiment of the present invention as a circuit diagram having a protective circuit for preventing an excessive positive voltage from being applied across a piezoelectric element. Referring to Fig.1, a battery voltage V cc is applied to an input terminal of a dc-dc converter 1 via an ignition switch Sw. The dc-dc converter 1 has its output terminal connected to an anode of a first thyristor SCR1 via a first choke coil Lc. The first thyristor SCR1 has its cathode connected to a piezoelectric element PZT. Thus, when the thyristor SCR1 is rendered conductive with an external control signal, an electric charge is charged in the piezoelectric element PZT. On the other hand, the piezoelectric element PZT is connected to an anode of a second thyristor SCR2 having its cathode connected via a second choke coil Ld to an input terminal of the dc-dc converter 1 and to the ignition switch Sw. When the second thyristor SCR2 is rendered conductive with an external control signal, the electric charge stored in the piezoelectric element PZT is discharged into the battery V cc through the second thyristor SCR2, the second choke coil Ld and the ignition switch Sw. A positive excess voltage detection unit 10 is arranged between the choke coil Lc and the thyristor SCR1. An output signal of the positive excess voltage detection unit 10 is fed to a positive excess voltage control unit 20, output terminals of which are connected to nodes across both ends of the choke coil Lc. The positive excess voltage detection unit 10 is made up of resistors R1 and R2, a Zenor diode ZD1 and a transistor Q1. The resistor R1 has its one end connected to an anode of the thyristor SCR1, while having its other end connected to a junction node of the resistor R2 and the base of the transistor Q1. The resistor R2 has its other end connected to a cathode of the Zenor diode ZD1, which has its anode grounded as shown. The transistor Q1 has its emitter connected to an anode of the thyristor SCR1. The positive excess voltage control unit 20 is made up of a resistor R3, a transistor Q2 and a diode D1. The resistor R3 has its one end connected to a collector of the transistor Q1, while having its other end connected to the base of the transistor Q2. The transistor Q2 has its emitter connected to an anode of the diode D1, while having its collector connected to a junction node of one end of the choke coil Lc and an anode of the thyristor SCR1. The diode D1 has its cathode connected to the other end of the choke coil Lc. The operation of the circuit shown in Fig.1, is as follows: The voltage of the battery V cc , supplied via the ignition switch Sw, is raised to e.g. 200 to 500 V by the dc-dc converter 1. Since a trigger signal for interrupting the fuel injection is connected to the gate terminal of the thyristor SCR1, the thyristor SCR1 is rendered conductive when the trigger signal is applied, so that the output voltage of the dc-dc converter 1 is fed to the piezoelectric element PZT via the choke coil Lc and the thyristor SCR1. When supplied with the voltage, the piezoelectric element PZT is deformed and increased in size or shape. The piezoelectric element PZT is employed as a valve for a fuel injection port of a fuel injection device and, when the voltage is applied, the piezoelectric element PZT is increased in size or shape to close the fuel injection port. That is, the piezoelectric element PZT is elongated in synchronism with the trigger signal applied to the thyristor SCR1 for closing the fuel injection port. On the other hand, the choke coil Lc and the capacitance of the piezoelectric element PZT make up an LC resonance circuit. Thus, there may be occasions where a voltage in excess of an output voltage of the dc-dc converter 1 is applied to the piezoelectric element PZT to exceed its withstand voltage. The Zenor diode ZD1 has a Zenor voltage e.g. of 900V. If the anode terminal voltage of the thyristor SCR1, that is the applied voltage of the element PZT, exceeds 900V, the base current of the transistor Q1 begins to flow and the collector current of the transistor Q1 flows via the resistor R3 into the base of the transistor Q2. Thus the transistor Q2 is rendered conductive so that the collector current flows. Thus the current flowing from the choke coil Lc through the thyristor SCR1 is now caused to flow as the collector current of the transistor Q2 and the current flowing through the thyristor SCR1 is reduced to a value lower than its holding current. As a result thereof, the thyristor SCR1 is rendered nonconductive to prevent an excess voltage from being applied to the piezoelectric element PZT to hold the voltage applied thereto just before the cutoff of the thyristor SCR1. Consequently, the fuel injection port is maintained in the state of valve closure. According to the present invention, since the excess voltage is detected at the anode of the thyristor SCR1, excess voltage detection may be realized without the piezoelectric element PZT being affected with e.g. the current leakage. Although the Zenor diode ZD1 is used as the positive excess voltage detection unit 10, the voltage divider made up of the resistors R1 and R2 suffices for detecting the excess voltage.

Claims: A fuel injection control device for an internal combustion engine in which an output voltage of a dc-dc converter (1) for raising a power source voltage (Vcc) supplied thereto via an ignition switch (Sw) is applied to a piezoelectric element (PZT) through a first choke coil (Lc) and a first thyristor (SCR1) controlling timing of charging the piezoelectric element (PZT), comprising: a positive excess voltage detection unit (10) for detecting a positive excess voltage at an anode terminal of said first thyristor (SCR1), and a positive excess voltage control unit (20) for shortening both ends of said first choke coil (Lc) under control by an output signal of said positive excess voltage detection unit (10). A fuel injection control device according to claim 1, wherein said positive excess voltage detection unit (10) is arranged between said first choke coil (Lc) and said first thyristor (SCR1), and said output signal of the positive excess voltage detection unit (10) is fed to said positive excess voltage control unit (20). A fuel injection control device according to claim 2, wherein the positive excess voltage control unit (20) is arranged in parallel to said first choke coil (Lc) and comprises a resistor (R3), a transistor (Q2) and a diode (D1), and the transistor (Q2) is rendered conductive due to said output signal supplied by the positive excess voltage detection unit (10) to shorten said both ends of the first choke coil (Lc).

Inventors: AKAKI MOTONOBU, MITSUYASU MASAKI, OOTA NOBUYUKI, SAWADA DAISAKU, YAMADA YASUTOSHI, AKAKI, MOTONOBU, MITSUYASU, MASAKI, OOTA, NOBUYUKI, SAWADA, DAISAKU, YAMADA, YASUTOSHI
Applicants: AISIN SEIKI, TOYOTA MOTOR CO LTD, AISIN SEIKI KABUSHIKI KAISHA, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1994
Main classification: F02D41
IPC codes: F02D41, F02M51, H01L41